Sudden loss of back light on iPhone 6

So after a couple days of my phone acting funny and continuing to fail calls, my screen went black. at least it appeared that way. its like it froze for a second and just went dark. After talking to apple support, we accomplished nothing. I did a soft reset, then restored it back to factory setting and nothing. I had an older iPhone 6 that had a perfectly good, working condition screen. Swapped them out and still the same thing. If i hold a light to the screen a certain way I can see some of the icons. I am curious what could cause this to happen all of the sudden. The screen is fully functional minus the lighting. Has anyone else experienced this. I'm assuming it a hardware issue but hoping for better luck.

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